Interest has been shown recently in the use of intermetallic compounds (such as aluminides, silicides, germanides, etc.) and refractory compounds (such as nitrides, borides etc.) for engineering use. Many of these compounds exhibit exceptionally high melting points, chemical inertness, and enhanced strength at elevated temperatures. These compounds represent new opportunities for technological materials advancement important to energy production and conservation, high speed aircraft, military systems, and chemical process industries.
However, methods of production and fabrication of these compounds have proven to be difficult. For example, the formation of many of these compounds is extremely exothermic, making containment during synthesis a considerable problem. Additionally, many of these compounds exhibit high melting points together with high reactivity in the liquid state, complicating conventional crucible melting practice that otherwise might be used in their manufacture. Furthermore, the low ductility exhibited by such compounds as fabricated by conventional casting techniques makes subsequent deformation processing virtually impossible. Although some of these compounds have been formed by the use of powder metallurgy techniques, the production of suitable powder material has proven time consuming, expensive, and hindered by crucible contamination and/or contamination from grinding operations. Plasma arc production has been used for the production of some of these compounds, but requires a high capital investment which adds to the cost of these materials.
There is a need for a method of producing these compounds in a manner that circumvents the aforementioned raw material production and component fabrication problems heretofore associated with these compounds.
Interest has also been shown for some time in dispersion strengthened metallic materials wherein the material typically comprises a metal or alloy (hereafter referred to as metallic) matrix having dispersoids distributed uniformly throughout for strength enhancing purposes. Such dispersion strengthened materials have been made by internal oxidation of the matrix to produce, for example, a metal matrix having a dispersion of fine oxide particles therein. Another method for the production of dispersion strengthened material has involved mechanical compaction of a mixture of the metallic powder and the dispersoid powders. Attempts have also been made to cast a metallic melt containing the dispersoids therein in a mold to form such materials.
Another method of making dispersion strengthened materials is the so-called "mechanical alloying" process of International Nickel Corporation wherein a blended mixture of matrix powder and dispersoid powder is mechanically attrited for long times to reduce the particle sizes and to force an intimate bonding of the two phases to form "composite" particulate. Still another method involves mixing powdered components followed by pressing and sintering.
There is a need for a method for making dispersion strengthened materials from a precursor material that can be readily treated to form the desired dispersoids in-situ in the metallic matrix.
The so-called XD process developed by Martin Marietta Corporation represents one attempt to provide such a method. The XD process forms second phase dispersoids (e.g. titanium diboride, titanium carbide, etc.) in-situ in a metallic matrix (e.g. aluminum matrix) as described, for example, in U.S. Pat. Nos. 4,710,348, 4,772,452, 4,751,048, 4,836,982, 4,915,905, and 4,915,908.
Gas atomization is a commonly used technique for economically making fine metallic powder by melting the metallic material and then impinging a gas stream on the melt to atomize it into fine molten droplets that are solidified to form the powder. One particular gas atomization process is described in the Ayers and Anderson U.S. Pat. Nos. 4,619,845 wherein a molten stream is atomized by a supersonic carrier gas to yield fine metallic powder (e.g., powder sizes of 10 microns or less). Anderson U.S. Pat. No. 5,073,409 and 5,125,574 describe high pressure gas atomization of a melt in a manner to form a thin protective refractory nitride surface layer or film on the atomized powder particles. The '409 patent uses an atomizing gas, such as nitrogen, that selectively reacts with an alloy constituent to form the protective surface layer. The '574 patent uses an inert atomizing gas and a reactive gas contacted with the atomized droplets at a selected location downstream of the atomizing nozzle to form the protective layer. Various prior art techniques for forming protective layers on atomized powder by reacting a gaseous species with the melt, or a component of the melt, are discussed in these patents.
It is an object of the present invention to provide a method of making atomized powder particles having a refractory or intermetallic compound formed therein by gas atomizing a metallic melt under melt temperature and atomizing gas reactivity conditions effective to form the desired compound throughout the atomized particles.
It is another object of the invention to provide a method of making atomized powder particles having a supersaturated solid solution of a dispersoid-forming species present therein by gas atomizing a metallic melt under melt temperature and atomizing gas reactivity conditions effective to achieve superequilibrium concentrations of the species in the atomized particles.
It is still another object of the present invention to provide a method of making a dispersion strengthened material wherein metallic powder particles supersaturated with a dispersoid-forming species by reactive gas atomization are heat treated to react the species with the host metallic material to form fine dispersoids in-situ in the metallic powder.